Signaling by the Wnt family of secreted glycolipoproteins is one of the fundamental mechanisms that direct cell proliferation, cell polarity, and cell fate determination during embryonic development and tissue homeostasis. As a result, mutations in the Wnt pathway are often linked to human birth defects, cancer, and other diseases. A critical Wnt pathway is the canonical Wnt signaling pathway.
The transcriptional co-activator Beta-Catenin is the molecular node in this pathway. In the absence of Wnt, cytoplasmic Beta-catenin protein is constantly degraded by the action of the Axin complex, which is composed of the scaffolding protein Axin, the tumor suppressor adenomatous polyposis coli gene product (APC), casein kinase 1 (CK1), and glycogen synthase kinase 3 (GSK3). CK1 and GSK3 sequentially phosphorylate the amino terminal region of Beta-catenin, resulting in Beta-catenin recognition by Beta-Trcp, an E3 ubiquitin ligase subunit, and subsequent Beta-catenin ubiquitination and proteasomal degradation.
This continual elimination of Beta-catenin prevents Beta-catenin from reaching the nucleus, and Wnt target genes are thereby repressed by the DNA-bound T cell factor/lymphoid enhancer factor (TCF/LEF) family of proteins. The Wnt/Beta-catenin pathway is activated when a Wnt ligand binds to the seven-pass transmembrane Frizzled (Fz or Fzd) receptor and its coreceptor, low-density lipoprotein receptor-related protein 6 (LRP6), or its close relative LRP5. The formation of a likely Wnt-Fz-LRP6 complex, together with the recruitment of the scaffolding protein Dishevelled (Dvl), results in LRP6 phosphorylation and activation and the recruitment of the Axin complex to the receptors. These events lead to inhibition of Axin-mediated Beta-catenin phosphorylation and thereby to the stabilization of Beta-catenin, which accumulates, travels to the nucleus to form complexes with TCF/LEF and activates Wnt target gene expression.
Given the critical roles of Wnt/Beta-catenin signaling in development and homeostasis, it is no surprise that mutations of the Wnt pathway components are associated with many disorders and diseases. McDonald et al. 2009 Dev. Cell 17: 9-26. Association of deregulated Wnt/Beta-catenin signaling with cancer has been well documented, particularly with colorectal cancer. Polakis 2007. Curr. Opin. Genet. Dev. 17: 45-51. Constitutively activated Beta-catenin signaling, due to APC deficiency or Beta-catenin mutations that prevent its degradation, leads to excessive stem cell renewal/proliferation that predisposes cells to tumorigenesis. Indeed, APC deletion or Beta-catenin activation in stem cells is essential for intestinal neoplasia. Fuchs 2009 Cell 137: 811-819.
Mutations in Beta-Catenin, alterations of levels and cellular compartmentalization of Beta-Catenin, and other aberrations of the Wnt/Beta-Catenin pathway are thus involved directly or indirectly with many diseases. In some Beta-Catenin-related cancers, Beta-Catenin is required for tumor growth but is not amplified, over-expressed or mis-localized.
There exists the need for treatments related to Beta-Catenin-related diseases.